Emergency response systems—also known as 911 systems in the United States and Canada, but known by other numbers elsewhere—cannot only rely on the caller correctly relying the location of an emergency to dispatchers. Callers may be lost, too incapacitated to provide address details, too young to know such details, be outdoors in a location without a definitive street address, or simply have forgotten in the stress of the emergency.
More modern emergency response systems (for example, Enhanced 911 or E911 in the United States and Canada) include the capability to send a caller's location to emergency dispatchers automatically. A database matches calls from landlines with the known address of that landline. However, the database must be kept up to date to be effective. As well, large office telephone systems may only indicate one central location, even though there are many extensions that could be called from.
Cellular telephones do not have fixed addresses to store in a database. Instead, cellular telephones may be located by triangulating such devices using the cellular transmission towers they are within range of. For example, if a device is within the range of three transmission towers, the device must be within the intersection of the coverage areas of these three towers. However, triangulation is imprecise, particularly in rural areas where towers are sparse and may have wide coverage areas.
More modern cellular telephones, such as smartphones, include components to report their location using satellite-based navigation systems such as the Global Positioning System (GPS). However, GPS relies on triangulation of signals from orbiting satellites, so is most accurate outdoors, away from nearby buildings that attenuate or distort weak GPS signals form the satellites.
Smartphones may also make calls using voice-over-Wi-Fi services, or supplement GPS location capability using other available signals such as Wi-Fi and Bluetooth. (Bluetooth is a registered trademark of Bluetooth SIG, Inc.) However, such systems may also rely on databases that must be kept up to date and can improve precision only in areas where they are available. For example, one recent study found an overall location accuracy of about 120 meters, even with all available sensors contributing.
In addition, all such location systems locate a caller based on the latitude and longitude of the calling device, and cannot compute a caller's exact elevation. As a result, emergency responders often have difficulty finding callers, particularly in large multi-floored buildings. For example, while a caller could summon help to a twenty-story office building, emergency responders would have much more difficulty locating the correct floor and office suite within the building.